The present invention relates to vapor phase corrosion inhibiting compositions, and more particularly to dry powder inhibitors specifically formulated to provide corrosion protection of metal in recessed areas, e.g. post-tensioning cables inside tubes.
Vapor phase corrosion inhibiting materials are utilized in a variety of applications for protecting metal from corrosion. One such application is a method of prestressing concrete structures, known as post-tensioning. Post-tensioned concrete systems have been used for decades in the construction of bridges, elevated concrete slabs for parking ramps and garages, and in flooring, walls and columns of commercial buildings. In this form of prestressing, cables, strands, bars, or other members of high strength steel are installed at a job site, usually housed in sheathing or tubes that prevent the steel from bonding to the concrete. After the concrete cures, the steel members are stretched by hydraulic jacks. The tensioned members act upon the concrete slab or other structure to place it in compression, considerably improving the capacity of the structure to withstand tensile and bending forces.
A persistent problem with post-tensioned structures and systems is corrosion of the metal members, particularly in environments involving exposure to salts and other de-icing materials, acid rain, airborne salts in locations near the ocean, and high humidity. If undetected or untreated, corrosion can weaken tensioned members to the point of breakage. In typical post-tensioned structures where the cables or other members are not bonded to the surrounding concrete, breakage of a tensioned member can create a risk of serious injury and property damage.
A variety of solutions have been directed to the corrosion problem. For example, U.S. Pat. No. 5,840,247 (Dubois et al.) discloses a process for protecting the tendons embedded in housings by drilling holes in the housings and injecting a corrosion inhibiting liquid solution into the housings while applying a high power pulsating wave to enhance penetration.
U.S. Pat. No. 5,460,033 (VanderVelde) describes processes for corrosion evaluation and protection of unbonded cables. Holes are drilled in the concrete to expose the tendons, and a dry non-corrosive gas is passed through the conduits enclosing the tendons. The patent notes that if the evaluation of the gas indicates a humidity above sixty percent, corrosion will ensue. The humidity preferably is maintained below forty-five percent, by injection of dry nitrogen gas as needed.
U.S. Pat. No. 3,513,609 (Lang) shows tendons coated with a polymeric material such as TEFLON (brand name) polymer or an epoxy resin containing up to twenty-five percent finely ground TEFLON polymer. The tendons are coated with a lubricating grease before they are covered with the plastic.
U.S. Pat. No. 4,442,021 (Burge, et al.) is drawn to a corrosion protection coating of cement containing up to ten percent corrosion inhibitors. The mixture is applied onto the metallic tendons before their enclosure.
U.S. Pat. No. 5,770,286 (Sorkin) describes a corrosion resistant retaining seal for end caps. The cap, formed of a polymeric material, contains corrosion resistant material inside the cap. The cap is intended to create a water-tight seal. The patent also describes an “ice pick” method of making a hole in the plastic sheath and injecting grease into the sleeve to displace water and prevent corrosion.
U.S. Pat. No. 5,540,030 (Morrow) describes injecting a polyurethane resin into the housing to displace water and air and prevent corrosion.
While the foregoing approaches are acceptable for a variety of applications, none of them is particularly well suited for providing corrosion protection for large scale systems in which the reinforcement members may have considerable length, e.g. exceeding one hundred feet. Drilling holes for injecting anti-corrosive grout or oil becomes prohibitively expensive and time consuming, and corrosion of longer lengths of tensioned members is not adequately addressed by end caps or similarly restricted features. Coating tensioned members directly with anti-corrosive layers or films inhibits corrosion, but is not a practical approach for treating previously installed systems.
Accordingly, the present invention concerns structures, systems, and processes directed to one or more of the following objects:
(1) to facilitate corrosion protection of metal tension members having considerable length, without the need to drill multiple holes along the length of the members to be treated;
(2) to provide a process for treating tensioned reinforcement members in situ in preexisting structures, at low cost and minimal disruption to the structures;
(3) to provide a process particularly well suited for protecting reinforcement members (either before or after they are tensioned) enclosed in relatively tight tubes or sheaths, or having irregular or varying topographies or otherwise forming relatively small or deep voids where exposed metal surfaces are difficult to reach.